Multiple choice

A motorist completes the journey between A and B at a constant speed of 20 kmph and covers the return journey from B to A at a constant speed of 30 kmph. What was the average speed?

  1. 50 kmph

  2. 24 kmph

  3. 25 kmph

  4. 26 kmph

Reveal answer Fill a bubble to check yourself
B Correct answer
Explanation

Average speed = 2ab / (a + b) when distances are equal. Here 2(20)(30) / (20 + 30) = 1200 / 50 = 24 kmph. This is the harmonic mean, not arithmetic mean (25 kmph). The formula derives from total distance (2d) divided by total time (d/a + d/b).